Well, I cannot fully agree with that. Cyrille asked:



I think, this question isn't fully answered by meantioning, that with TTL also a current is flowing or that it relies on current rather than voltage.

When you have a "voltage output", then this means that the driver provides a certain stiffness, means the driver is able to keep the output signal (voltage) constant, even when the load (e.g. load impedance) relevantly changes. The same, when you have a "current output". Then the current driver also provides a certain stiffness, means the driver is able to keep the output signal (current) constant, even when the load (impedance, compliance voltage) relevantly changes.

With TTL we definitely have a voltage signal, that is emitted, not a current signal, although some current is finally flowing. But the emitted signal must be a voltage signal, because it needs the stiffness of the voltage output driver, to guarantee that 10 or even 20 of such TTL-inputs can be driven by the same output at a time, without that the output signal undergoes relevant changes.

Even when also current is flowing when TTL circuitry transmits signals, it would be a huge difference if the TTL output would actually emit a "current signal", which is how I understand Cyrille's question. In such a case you could no longer put several inputs in parallel like you can do when emitting a "voltage signal". Using a true "current signal" would mean, that you have to connect inputs in series. So, each single gate would need two input lines to be properly connected to the environment, while with a "voltage signal" you need only one line per gate, because all gate inputs are automatically referenced to signal ground.

Kai